Secret signaling system



c. E. A'mlmsA SECRET SIGNALING SYSTEM Filed June 16, 1943 2 Sheets-Sheet l WINS QSQMQW Juy 239 3%@ c. E. ATM-Ns SECRET SIGNALING SYSTEM www Filed June 16, 1945 2 Sheets-Sheet 2 www@ Patented july 23,1946

SECRET SIGNALING SYSTEM Carl Edward Atkins, Evanston, Ill.,` assigner to Tung-Sol Lamp Works, Inc., Newark, N. J., a

corporation of Delaware Application J une 16, 1943, Serial No. 490,986

(Cl. Z50-6) Claims.

My present inventionrelates to secret signalling systems of the general type disclosed and claimed in my copending application Serial No. 443,898, filed May 21, 1942, and more particularly to the specic type of such secretl signalling system disclosed and claimed in another of my copending applications, namely Serial No. 468,- 345, filed December 9, 1942, on the system of which the present application is an improvement.

In my present system, as in those of my prior applications, secrecy is obtained by the use of reciprocal modulation devices, signal energy introduced at one station serving to cross-modulate or scramble the signal transmitted from the other station. As in the system of Serial No. 468,345, communication carriers between two stations are simultaneously frequency-modulated and the entire system including the apparatus at both communicating stations and the space therebetween constitutes an end-to-end oscillator where the frequency of oscillation undergoes several changes but is fundamentally determined by the relationship ,between the various elements of the system as hereinafter described.

The present invention is primarily concerned With an improved means of and method for detecting at one station the frequency modulation introduced at the other station without interference with frequency modulation introduced at the local station. The invention permits of a larger frequency sweep, that is frequency deviation or depth of modulation, than in the case of the system of Serial No. 468,345 and also results in a simpliiication of the equipment at each station.

For an understanding of the invention reference may be had to the accompanying drawings, of Which- Fig. 1 is a block diagram of the communication apparatus at either of two communicating stations;

Fig. 2 is a circuit diagram showing in more detail the special lter and the signal originating apparatus of Fig. 1; and

Fig. 3 is a circuit diagram of an alternative operating frequencies. The apparatus of Fig. 1

will rst lbe described in general terms applicable tothe arrangement at either station.

In Fig. 1 a receiving antenna 2 is arranged to supply energy to a combined receiver and frequency converter 4. The receiving portion of unit 4 should be so constructed as to be able to pass a relatively wide band of frequencies. The particular width of the band passed, as well as the sensitivity and characteristics of the receiving apparatus is of course a matter of design, but for convenience in description and in order to give a concrete example of the operation of the system, the width of the band passed may be taken as or 30 kc. The converter portion of unit 4 is arranged to change the received signals,

which are of the order of la few mega cycles, to an intermediate frequency, say of 100 kc. The output of unit 4 isltransmitted through a radio frequency transformer 6, having suitable bandpass characteristics, -to a limiter stage 8. The limiter 8 may be. of any type known to the art, such as a saturatedV pentode or a pair of diodes. If desired, a limiter circuit of the type disclosed in my earlier led application Serial No.V 425,843, filed VJanuary 7, 1942, could be utilized herein as unit 8. The output from limiter 8 is fed to onel pair of input terminals of a balanced modulator I0; the other pair of input terminals of which receive energy from a special filter I2 which is interposed between balanced modulator I0 and a unit 20 which is the point of origin of the signal to be transmitted. Filter I2 and unit 2l] are shown in more detail in Fig. 2. Balanced modulator I0 may be of any type known to the art but preferably is of the type disclosed and claimed in my copending application Serial No. 457,807, filed September 10, 1942, which insures suppression of the beating frequencies inthe output circuit. The energy f from limiter 8 heterodynes in balanced modulator arrangement suitable for use as the detecting The corresponding elements of the apparatus VIll with the energy delivered from filter I2 so y that sum and difference frequencies appear at the output terminals of unit I 0 and are impressed upon a radio frequency transformer I4 which is a .band pass circuit passing the difference -frequencies only. Thus if the frequency of the energy from lter I2 is around 40 kc., the pass band of transformer I4 will center around 60 kc. Energy from transformer I4 appears at the input terminalsof a balanced'modulator I6. In balanced modulator I6, which is preferably of the same type as unitY I0, energy from transformer I4 heterodynes with energy of a frequency of corresponding elements to permitl of `different about l40 kc. supplied *froml unit 2D. The sum of oscillator 32h will be 140'kc.

3 and difference frequencies from unit I6 are delivered to a radio frequency transformer 22 having `band pass characteristics such as to pass only the sum frequencies to a frequency converter and transmitter unit 24. The frequency of the energy delivered to unit 24 is thus the same order of magnitude as that of the energy delivered by unit 4, that is, about 100 kc. Unit 24 converts the received energy by suitable heterodyning action to a signal frequency of several thousand kilocycles, amplies the energy and delivers it to a transmitting antenna 26.

The apparatus of Fig. 1, so far described, is

substantially the same as that of my copending Y application Serial No. 468,345. In the system of that application lter I2 was intended to delay the application to unit IU of the signal frequency variations originating in unit until such frequency changes had made the circuit of the two stations and appeared in the output from limiter 8. With filter I2 so designed, a detector unit at one station connected directly across the output of transformer I4 would respond only to frequency changes occurring in unit 20 at the other station. With such a connection of the detector unit, it is necessary, in order to eliminate all residual local frequency changes in the detector, to incorporate a high order of delay in filter I2 and this in turn requires that the depth of modulation, that is frequency deviation, be kept within reasonable limits to minimize a more or less erratic tendency of the system to jump frequencies, that is to oscillate at any one of several frequencies, when such high order of delay is introduced in filter I2. With the present system, in which a special detecting unit is employed and which is connected to the output circuits of both transformers I4 and 22, a reasonably large frequency sweep may be employed without the danger of the system jumping to a new idling frequency and without even the smallest trace of locally produced frequency modulation appearing in the local detector. This will be apparent as the description proceeds.

In the detecting unit of Fig. 1 there is a multielectrode thermionic tube 28, such as a GSA?, of which one grid is energized by the output of transformer I4 and another grid is energized by the output of transformer 22. The output of tube 28 passes through a radio frequency transformer 29, which is tuned so as to pass the sum of the two frequencies applied to the control grids of tube 23. Energy passing through transformer 29 appears on a control grid of a second multi-electrode tube 30, which may be of the same type as tube 28, a second control grid of which is excited by an oscillator 32 of a frequency equal to the sum, under idling conditions, of the two beating frequencies applied to tube 28. An audiofrequency load impedance 34 is includedin the output circuit of tube Yand the voltage across this impedance is fed through a blocking condenser 36 to any suitable audio-frequency amplifier and reproducer (not shown).

In order to simplify the explanation of the system so far described, specific frequencies will be assigned to the various oscillators, and the unitsY at one station, say station A will be identified 4by the subscript a and those at the other station, say station B, by the subscript 1). j'Assume the idling frequency of unit 20a toibe 40 kc., that of unit 20bto be 60kc., then the frequency of oscillator 32a will be 160 kc. and that Also transformer 29a will respond to frequencies in the neighbor- 'turned to its idling frequency of 40 kc.

hood of 160 kc. and transformer 29h to frequencies in the neighborhood of kc. On this assumption, while the system is idling, transformer Ilia feeds 60 kc. energy to a grid of tube 28a while transformer 22a feeds 100V kc. to the other control grid of tube 28a. Transformer 29a` passes the sum of those frequencies kc.) to one control grid of tube 30a where it beats with energy of the same frequency from oscillator 32a applied on the other control grid of tube 30a. Thus zero beat obtains and there is no audio frequency output across load impedance 34a. If now there is a frequency shift (which can be either of a discrete amount or can be at an audio frequency rate in the manner commonto frequency modulation systems) in unit 20a, this frequency change will be applied simultaneously to filter I2 and balanced modulator I6. Filter I2, as shown in Fig. 2, is composed of a plurality of adjustable network sections, which introduce a limited amount of delay inthe application to unit I0 of the frequency changes originating in unit 20.` By suitable adjustment of the network of filter I2, either manually or automatically, it is possible to cause half of the frequency deviations originating in unit 20 to take place at the terminals of transformer I4 while the other half of the total deviation takes place at the terminals of transformer 22. This is possible because frequency deviations at transformer I4 are in opposite direction to those occurring in transformer 22. Specifically, assumel that the frequency of unit 20a departs from its idlingy frequency of 40 kc. and is at the moment 41 kc. Then, if filter I2a is properly adjusted, the beating frequencies in unit Ia are A100-kc. and 40.5 kc. and hence the frequency at the. output of transformer IIIa-`wil1 be 59.5 kc. The addition in balanced modulator IBa of this 59.5 kc. to the 41 kc. produced by unit 20a. will yield 100.5 kc. in the output of transformer 22a. Thus the frequency changes applied to the control grids of tube 28a due to frequency deviations in unit 20a, are equal and opposite, and the energy applied through transformer 29a will. not depart from the 160 kc., or idling value.

This frequency shift of unit 20a from 40 kc. to 41 kc. will, however, be detected at station B as the 100.5 kc.. at the output of transformer 22a, after conversion to a high frequency in unit 24a, transmission to station VB, reception and frequency conversion in unit 4b of that station will appear at the input terminalsv of. balanced-modulator Ib. If unit 20h is operating .atits idling frequency of 60 kc., the frequency at the terminal of transformer Mb will thus be 40.5 kc. and that at the terminals of transformer 22h will be 100.5 kc. Hence tube 28h will pass the sum frequencies, namely 141 kc.,v through transformer 29h to. tube 30h where it will beat with energy of 140v kc.V from oscillator 32D, with the result that the 1 kc. increment of frequency in unit 20a. appears in the output of tube 30h for audio frequency detection.

Still vassuming that the oscillator' of 'unit 20h is at its idling frequency, energy of 100.5V kc. afterA conversion to a higher frequency at station B, transmission therefrom, reception and reconversion in unit 4a will appear at the input terminals of balanced modulator I0a. During`v this cycle, assume that oscillator of unit 20a has re- Because of ther delay inherent in filter I2`a, however; the frequencyA of the energy delivered thereby to unit IUa-will'still be 40.5 kc. `Hence the` frequency in thefoutput'fromltransformer M a will return to 40 kc. and that of the energy in the outputof transformer 22a will return to 100kc'. with fzer'o beat lstill obtaining jin the detector of station A. If, on the Vother hand, the 'frequency delivered by unitv 26a, insteadA of returning to its idling value, had increased to 42 kc., this would still not affect the :detector at station-A. Under 'these circumstances the frequency of the energy passed by filter I2 would be 41.5 kc., corresponding to the idling frequency plus one-half of the instantaneous-deviation, plus the delayed half of the former deviation. The frequencies applied to the `tube 28a.` would thus be` 59 kc. and `101 kc. instead of 60Ikc. and 100 kc., but their sum, namely 160 kc. in each case is the same and insures zero beat in the detector. y

The above description, using assumed discrete frequency changes, has been given as an aid to the understanding ofthe operation of the system. Actually, of course, the frequency changes taking place in the system arercomplicated anddofnot occur in discrete steps. Y Y z From the above description it will be `apparent that frequency deviations introduced at one station dofnot actuate the detector at thatstation but are detected at the other station and simultaneous two-way communication maybe had with the signals introduced at one stationvserving to camouflage or scramble the signals introduced at the other station. f, 'l i 1:-

-Reference may nowV be had to Fig.. 2 .which shows apparatus suitable for use as units I2' and 2D of.;Fig. 1. Unit 20, which in the particular embodiment of the invention il1ustratedis,ar ranged for either telegraphic or telephonie communication, includes an oscillator 38 the idling frequency of which, for the numerical values heretofore. assumed, will be 40 kc. for unit 20a and .60 kc. for .unit 20h. A reactance tube 40 is arranged to cause deviations of the frequency of oscillator` 36in response to audio frequency voltages applied to a controll grid thereof from a source 42 ofcapricious Voltage or-from an am plier 44.. When telegraphic communication is to be had, the sourceof ycapricious voltage 42 is connected in series with a key `46 across the input terminals of tube 40 by means of a switch arm 48 biased by a spring 50 into circuit closingposition. i With two-way telegraphic communication, the` frequency deviations introduced at the two stations by the operation of the key 46 at each will mask each other and render deciphering difficult, if not impossible, by an unauthorized interceptor of the :.radiated energyv fromv either station. `Where one-way communication only is desired, the' key 46 of the receiving station will be shortedby arswitch .1,52- to apply the voltage from source 42 continuouslyto-tubell. Thus random fluctuations of frequency introduced by unit 20 ofthe receivingfstation will, camouflage the message introduced into the syste1nat fthe signalling station.

The apparatus of Fig. 2 used for telephonic communication includes a solenoid v54 -,which, when'energized,` moves switch arm 48` against its spring bias into a. position to open the circuit of the source 42 and impress the. output of am.- plil'ler 44' across the reactance tube. Solenoid 54 is ehnergize'd whenever speech or vother audible signal is available at the microphone 56 connected to the amplifier 44. As shown, the sole- 58 will pass current only when current Hows through `the'vdiode 60 asthe result'of4 audio frequency voltages appearing at the output. of amplier 44. During speech communication, switch 52 is closed so that during the intervals between words and consequent closure of the circuit of source 42, that source may interject random fre.- quency deviation for scrambling the message from theother'station.

vFilter l2, as shown in Fig. 2, includes a series of like sections or vcells 64, in this case two in number, asection vor cell 66, a resistance-condenser cell I68 and a limiter 10. Each'cell 64, of which as -many may be employed as are found necessary in any particular set-up, comprises an inductance and anxadjustable condenser in series and an adjustable resistance in shunt. By means of the adjustable. condenser of veach cell 64, each cell Aclan be operated'at or near the resonant frequency of the signal from unit 20 and by means of the adjustable shunt resistor the output of each cell 64 can be held. constant while the mag-l nitude of phase shiftis controlled. Cell66 comprises a simple resonant circuit of inductance and capacity and serves to provide a higher output voltage to make up for the inevitable `attenuation caused by passage through the sections 64. The last section serves to introduce a relatively constant delay compared to .the frequency shift. Cell 68 feeds to limiter 10, which is nonreactive and ls necessary because of the unavoidable amplitude change occurring with the phase shift- When the frequency kis changed.l Limiter 1 V0 in turn delivers the delayed signal energy to balanced modulator l0 of Fig. 1. 'I'he adjustment ofthe filter sections'can be eifectedfmanually or automatic means such as disclosed inmy prior application Serial No. 443,898 could be provided. Y In practicethe main adjustmentscould bemade'duringmanufacture of the equipment, with only minor slight adjustments required when the apparatus was in use, such other adjustments being necessary only to correct for minor changes in thevalinement of the` apparatus at theother stationand'to allow for changes in signalling distance; V,

Although the diagrammatic'circuit of filter l2 as shown in Fig. 2 and as above described, does not oli1fer...fr`o'm Athe corresponding delay filter illusf trated in my earlier system Serial No. 468,345, in practic'e`,'however, substantially fewer cells 64fare required in the special filter of the present system than in that of the earlier system. Also, because of the fewer number of cells,.amplilcation stages and additional limiters lwhich in practice are required with the filter lof the earlier system can be Aomitted in the present system with consequent substantial simplification of equipment.

' Instead of the sp'ecic detecting circuitvof Fig. 1, a circuit such as .that disclosed in Fig. 3 may b e used. Thecircuit of Fig. 3 `includes the tube` 28 with itscontrol grids connected as in Fig. 1 to the output circuits of transformers I4 and 22. The output circuit of tube 28` includes a circuit 12.

For theval'u'es heretofore assumed, circuit 12a is tuned to kc. 'and circuit 12b is tuned to 140 kc. A second circuit 14 tunedto the same `frequency is magnetically coupled to circuit 12 and a. center tap on `the`coi1 15 is conductively coupled as well frequency energy appearing across series-con- 'I nected load resistors .8.1. and. 8.2. as-.a.re.S1llt 0f frequency excursions; applied.` to l the. circuitare passed, onto any suitable. audio., frequencyap.- paratus. as inthe case ofthe detector circuit-got Fig-.1.."`: j Theoperation of the system4 whenfusing the abovefdescribed detecting circuit; of, Fig. 3 is the same as? heretofore .described with referencel to Figs. l and 2. When the reactance-.tubes 40 o f unit-a. and. 20h. are excited by speechor by operation of the telegrapnkey, thej equipments at stations A and B willcooperate.togetherinthe production of mutual cross modulation. :.Since the. frequencyv modulation voriginating at,k station A, for example, will .not appear in the output from tubev 28a because of the? balancing outwaction previously described,V onlyrthev frequency excursions originating at station B- will appear across the@ load resistors 8| and `82 and be detected at station A. v

' From the above description it will be apparent that the present invention providesan improved method of and means for detecting at one station frequency excursions originating attherother'station in an'end-to-endoscillator system in which secrecy of communicationis obtained by mutual crossmodulation produced jointly by Vthe communicating stations. The improved* detecting system permits a relatively wide frequencyy sweep, o r depth" of modulationv without danger of a jumping of the idling frequency land withrelatively simple equipment.- Y l Various changes` and refinements could' of course be made to the'system of.y the present invention as vabove described without departing from the spirit ofthe invention. For example, known means'fo'r preventing frequency drift; of the oscillators 'provided for frequency conversion at each station could and probably wouldbe 'prof vided in practice. Although no means have been described or illustrated for` keeping V energy 'radiated' from the'transmitting antenna out of to said circuitg'of a frequency equal vto the sum of the frequencies of the excitation voltages applied to-the control grids,v and means connecting the controlfgrids of the tube at one station with energized elements ofthe equipment at vthe same sta,- tionwherethe frequency of theenergyis effected equally and oppositely by locally introduced fre'- quency modulation and. effected equally and in the same direction by frequency modulation introduced at the other station whereby the frequency of the energy in the outputfcircuitof the detector ofone station is effected-'only by Vfrequency modulation introduced yat. the,... other station. I

` 2. Ina station-to-station secret signaling sys-` tem ofthe type wherein secrecy is obtained by simultaneous frequency modulation and wherein eachvstation includesa receiver and a transmitter .interconnected-by a chainof units comprising two frequency mixers and twoiilters with one of said filters connected between said mixers and designed; to pass the difference frequencies fromone mixer to ,the other, and the otherof said'iilters connectedL to the output ofrsaidlast mentioned mixer and designed; to pass the sum frequencies, and. wherein eachvstation includes a local oscillatonV for; vdelivering vbeating Y frequency to' said mixers4 and .controllableL means for varyingv the frequency thereof, the improvement which. comprises meansinterposedY between said oscillator and; the first, mixer of said chain for suppressing half.. of the. frequency variationsof said oscillator andra, detector connected t0 the output of both of said lters and responsive to variations, in the sum. of: the frequencies. passedthereby. Y

3;. The. improyement. according, to claimV 2 wherein isaid. detector includes'a multi-electrode tube.` having; a control grid connected. to one of said Yfilters;and'azsecond. controL-grid connected to the other of said filters, a. circuit in the output circuit of said tube tunedto a frequency equal to. the. sum ofv the. frequencies passed by .said filters under idling. conditions, asecond multielectrode tubehaving a. pair of control grids,an oscillator'generating energy of a frequency equal tothe sumof the frequencies passed` by saidlters underidling conditionsand. connected to .one of the control grids of said last mentionedtube, and means for energizing the other control. grid. of the last mentioned tube from saidv` tuned circuit whereby f frequency.V deviations introduced .at one station may be detectedain they output. circuitv of said last mentioned tube at. the other station while frequency deviations producedv at one station do not affect the detector at the same station..

l4. The Vimprovement according. to. claim 2 wherein saiddetector includes a multifelectrode tubehavingfa control grid connected to one of saidl filters and a second control gridy connected to the'otherof said filters, arpair ofcircuits each tunedto a frequency equal to they sumof the frequencies passed bylsaid filters under idling conditionsand-magnetically and conductively coupled together, one of said circuits being connected in theoutput circuit of said tube and theother of said circuits feeding to a pair of diodes, and serially connectedload resistors operating in cooperation with` said: diodes` whereby audiofrequency. energy appears across the load resistors at .one station only when said controllable means at thewother-V station causes the frequency..V of .the energy A delivered .by said i oscillator to vary.

5. Inv a station-to-stationisecret. communication system of' thetype wherein. a. chain of serially'connected energy passing units ateach station form,'togetherfwith the: space between the stations, an-end-to-end oscillatory andk wherein the equipment vat each'k station includes an oscillator having a fixed idling frequency and includes controllable means for causingthe` oscillator to deviate from itsidling frequencywhereby secrecy may be obtained by simultaneous cross-modulation` of the communication carriers,.one unit at each station'passingfrequencies of theordervof thelidlingrfrequency of the. oscillator atv the other station .and another. unit at. eachV station .passing frequenciesof the order ofthe sum of the idling frequenciesof the oscillators at both stations, the improvement `which comprises a detector at each station vconnected to saidk two last'men'tioned units,` said. detector including meansV for comparing thesum of the frequencies `passed by said twounitswith the. sum of the frequenciespassed by saidl units underidling conditions, and. connections between the oscillator at each station and other units of the chain at that station arranged to cause deviations of the frequency of such oscillator to appear equally and oppositely in the units connected to said detector whereby frequency deviations of the oscillator at one station affect only the detector at the other station.

6. The improvement according t claim 5 wherein one of the units to which the oscillator is connected to deliver energy thereto is a mixer receiving also energy from the unit passing frequencies of the order of the idling frequency of the oscillator at the other station and passing energy to the unit passing energy of the order of the sum of the idling frequencies of the two oscillators and wherein the other unit to which the oscillator is connected is a mixer located in the chain in advance of the unit passing frequencies of the order of the idling frequency of the oscillator at the other station, the connection between the oscillator and said last mentioned mixer including an adjustable series of networks for suppressing half of the frequency deviations of the oscillator.

7. The improvementl according to claim wherein said detector at each station includes a multi-electrode tube having two control grids, each connected to one of said two units, an output circuit for said tube tuned to the sum of the frequencies passed by said units under idling conditions, a second multi-electrode tube having two control grids, and an oscillator having a xed frequency equal to the sum of the frequencies passed by said units under idling conditions, one of the control grids of said last mentioned tube being connected with said last mentioned oscillator and the other control grid of said last mentioned tube being connected to said output circuit whereby audio frequencies in the output of said last mentioned tube correspond with departures of the sum frequencies of said units from the sum frequencies under idling conditions.

8. The improvement according to claim 5 wherein said detector includes a multi-electrode tube having two control grids each connected to y one of said two units, a, pair of circuits inductively and conductively connected together, each tuned to ak frequency equal to the sum of the frequencies passed by said units under idling conditions, one of said circuits being connected in the output circuit of said tube, a pair of diodes and serially connected load resistors so connected with said second tuned circuit that the departure of the sum of frequencies passed by said units from the sum passed under idling conditions appear as audio frequency energy across said load resistors. i

9. In a system of the type wherein chains of energy passing units at the two stations together with the space between the stations comprise an end-tO-end oscillator in which the frequency of oscillation undergoes various conversions, additions and subtractions during passage of the energy from one station to the other and back again but is fundamentally determined by the constants forming the energy passing units, the method of secret signaling between stations which comprises simultaneously frequency cross-modulating the oscillatory system by so introducing frequency deviations at each station as to introduce twice as great a frequency deviation at a unit where frequency addition occurs as at a unit of the chain where frequency subtraction occurs and detecting at each station the frequency deviations introduced at the other station by determining the change in the surn of the frequencies passed by the two units.

10. The method of secret signaling between two stations equipped with apparatus constituting, with the intervening steps, an oscillatory chain which comprises simultaneously frequency modulating the communication carriers at the two staw tions by introducing local frequency variations equally and oppositely at separated points in the oscillatory chain and utilizing at one station variations in the sum of the frequencies at such separated points for detection of the frequency variations introduced at the other station.

CARL EDWARD ATKINS. 

